In electrophotography, generally, a surface of an electrostatic latent image bearing member is charged by a method such as corona discharge, followed by exposure by means such as a laser to form an electrostatic latent image. The formed electrostatic latent image is then developed with a toner to form a toner image. The formed toner image is further transferred onto a recording medium to obtain an image with high quality. For forming the toner image, typically, The toner containing toner particles (toner base particles) with an average particle diameter of from 5 μm to 10 μm are used, which are obtained by mixing a binder resin, such as a thermoplastic resin, with toner components such as a colorant, a charge control agent, a release agent, and then by subjecting the mixture to a kneading step, a pulverizing step, and a classifying step. In order to provide flowability and preferable electrostatic properties to the toner particles and/or to provide easy cleaning ability of the toner particles from surfaces of photoconductor drums, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner base particles.
Such a toner is desired to have excellent fixability at low temperatures, which can be properly fixed without heating a fixing roller as much as possible, in terms of saving energy and the minimization of devices. In the production of a toner having excellent fixability at low temperatures, however, binder resins having a low melting point or a low glass transition point, or release agents having a low melting point are often used. For that reason, there are disadvantages in which toner particles in the toner are easily agglomerated when it is stored at a high temperature.
If toner particles agglomerate, an image failure may occur as a result of adhesion of toner particles onto a developing sleeve or a photoconductor drum, or fogging may occur in the formed image as a result of charge failure of toner particles when an image is formed by using a toner containing agglomerated toner particles. If toner particles agglomerate, the charge characteristics of the agglomerated toner particles change in comparison with other toner particles that are not agglomerated. In this case, agglomerated toner particles may adhere to a location that is not related to the image to be output, leading to color spot in the fixed output image.
To cope with the problem described above, a toner composed of toner particles having a core-shell structure in which core particles including a binder resin having a low melting point are coated with shell layers composed of a resin having a glass transition point (Tg) higher than that of the binder resin included in the core particles is used to improve fixability in wide range of temperatures, to improve storage stability at high temperatures and to improve blocking resistance.
The following toner particles have been proposed as toner particles having the core-shell structure as described below. The toner particles are toner particles with an amino group-containing compound attached as an external additive on the surfaces of toner base particles. The toner base particles are formed in the following manner. First, emulsified and dispersed fine particles of a binder resin and colorant fine particles are aggregated in an aqueous medium, followed by fusing of the fine particles together to form core particles. Then, fine particles of a positively chargeable charge-control resin containing quaternary ammonium salt-containing acrylate units, which are emulsified and dispersed, are added to the aqueous medium containing core particles. Thereafter, the positively chargeable charge-control resin fine particles are fused to the surfaces of the core particles to form shell layers, thereby obtaining toner base particles.
The above-described toner particles have shell layers composed of positively chargeable charge-control resin fine particles and an external additive that is an amino group-containing compound, and therefore when an image is formed by using the toner particles, the toner particles are charged at a desired charge value to suppress fogging in the formed image. However, when the above-described toner particles are used, the external additive may be embedded in toner base particles or detached from toner base particles if printing is carried out at a low coverage rate for a long time and toner particles are stirred in a developing unit for a long time. Consequently, toner particles may not be charged at the desired charge value. In this case, it is difficult to suppress fogging in the formed image.
In the above-described toner particles, the molar ratio of quaternary ammonium salt-containing acrylate units in the resin forming shell layers is only about 1%, and therefore toner particles may not be charged to a desired charge level in a short time, depending on the state of formation of the shell layers. In this case, color spot or fogging tends to occur in the formed image.